1. Field of the Invention
The present invention relates to an improvement of photolithography processes, and more particularly to a method of optical correction for improving the pattern shrinkage caused by scattering of the light.
2. Description of the Related Art
Photolithography processes include an exposure step in which a desired pattern is transferred onto a photoresist coated on a substrate by use of a photomask. The photoresist is removed selectively depending on the intensity of the light imaged thereon. When the optical path of light passing through the photomask deviates from the desired optical path, the pattern size obtained on the photoresist does not correspond to the size of the pattern on the photomask. This is known as the proximity effect.
The proximity effect can be partially compensated by amending the patterns on the photomask. For example, if it is known that an image formed on a photoresist layer is narrower than the pattern on the photomask, the pattern on the photomask may be designed wider than the desired size of the image. Data sets of photomasks used in a photolithography process, including amendments, can be stored as a database in a computer and accessed by different users.
In addition, pattern shrinkage caused by scattering of the light during the exposure step is also a problem, especially when the size of devices is tiny, for example, 0.5 microns or even less than 0.25 microns. Referring to FIG. 1, a top view of the pattern shrinkage after photolithography processes is shown. In the left side of FIG. 1, the length of pattern 10 on a photomask is b, and the distance between the edges of the two patterns 10 is a. The patterns transferred onto a photoresist layer by photolithography are shown in the right side of FIG. 1, which is marked 12. Because of the effect of scattering of the light at the opaque edges of the photomask during exposure, the length of patterns on the photoresist layer is shortened from b to b', i.e. b&gt;b'.
Methods have been developed to solve the problems caused by scattering of light. For example, serifs or hammerheads are added in the edges of the patterns on photomasks so that the areas of the patterns are larger than before. Thus, the exposed area on photoresist is decreased, and the pattern shrinkage phenomenon caused by scattering of the light is therefore reduced. The `serif` represents a small block added on the center or the edges of the patterns, while the `hammerhead` represents a connection (fusion) of two serifs.
Referring to FIG. 2, a top view of a corrected pattern on a photomask after photolithography processes is shown. First, patterns 10 are formed on a photomask (not shown). In addition, serifs 15 or hammerheads 17 are added to the edges of the patterns 10 as shown in FIG. 2. The patterns are transferred onto a photoresist layer (not shown) by photolithography processes so that an accurate patterns 12 are formed. As described above, the serifs 15 or hammerheads 17 are added to the edges of the patterns 10 on a photomask so that the areas of the patterns on the photomask are larger than before. Thus, the exposed area on photoresist is decreased, and the pattern shrinkage phenomenon caused by scattering of the light is therefore reduced. Accordingly, the length b of the pattern on the photomask equals to the length b' of the pattern on the photoresist layer, i.e. b=b'.
However, the formation of these photomasks as described above is complicated because the additional serifs or the hammerheads must be formed individually by electron beams. If there are many serifs or hammerheads needed in a photomask, the time or the money for fabricating a photomask is increased. Furthermore, it is observed from results of experiments that when the space between two patterns is close to a multiple of the wavelength, a standing wave effect can occur. This makes the patterns formed on the photoresist wavy. Further, the use of serifs or hammerheads does not entirely resolve the scattering of the light.